Conventionally, a sensor system which may be used for monitoring a value of a parameter, such as temperature, for example, may comprise a plurality of sensor units, each containing one or more sensors, and a central unit for collecting, and possibly processing, data from the sensor units. To this end, sensor units are typically arranged for transmitting data items to the central unit, either by wire (using copper and/or optical fiber) or wirelessly (using for example RF, infrared, magnetic, and/or acoustic transmission).
The data items may be data packets containing values obtained by the sensors. Usually, these values are digital values, for example, sampled values of an analogue physical phenomenon such as temperature. In such a case, the data items will contain data samples. The data items may also be constituted by a single (digital) value, such as a single byte representing a meter reading. In the present document, the word “sample” may therefore also refer to a data item transmitted by a sensor unit. Such a “sample” may thus contain one or more sampled values. The number of “samples” transmitted per time unit (e.g. second, hour or day) may then be referred to as “sample rate”. In other words, the term “sample rate” as used here represents the number of data items transmitted per unit of time and may therefore also be referred to as data item rate.
A data item rate may be determined by the sensor units so that the sensors are not polled. Instead, the sensor units determine the rate at which they send data items. This rate may be defined by a timer or clock which is internal to the sensor units so as to produce sensor values at a (more or less) fixed rate, but may also be determined by the values measured by the sensors: some sensor units may only transmit a data item when a sensor has detected a change in the sensed value. In either case, the central (or processing) unit can typically not influence the rate at which it receives data items.
In a sensor system, it is important to be able to detect abnormal behavior of the sensor units. That is, it should be detected when a sensor (or sensor unit) fails, or when a sensor detects an unusual condition. In a sensor system, it is desirable if the sensor units are monitored so as to automatically detect abnormal conditions. In a sensor system in which the sensor units produce data autonomously, that is, in which the sensor units are not polled, this is complicated by the fact that the monitoring unit, typically the central (or processing) unit, depends on the data items received. An absence of received data items over a prolonged period of time may be indicative of an abnormal condition. However, it is a problem of the sensors units known in the art that they are not suited to decide automatically whether an abnormal condition has occurred.